Friday, September 6, 2013

Prime Focus Problems, Can't Reach Focus on Meade Refractor

I'm trying to take the next step with my astrophotography, and that means utilizing my Meade 285 2.4" 900mm refractor. In order to do this, I need a way to mount my DSLR onto the telescope. I've heard of t-rings and t-adapters, and it seems pretty straight forward. The t-ring connects to the DSLR where the lens would normally attach - and therefore, you need to get one that corresponds to your type of camera (in my case Canon EOS).

Ok, so the t-ring is specific to your camera, but the t-adapter is a universal (I think?) thread dimension. The other side fits into a 1.25" eyepiece holder.

T-ring and t-adapter assembled on Canon Rebel XT

T-ring for Canon EOS type, and universal 1.25" t-adapter

T-ring fits snug into lens coupling, and locks with the lens release

Ok, so everything was hunky-dory, and I got my t-ring and t-adapter connected to my camera. Last night, I took my telescope out to try to focus on Venus. I couldn't get Venus to focus. I tried taking my focuser tube all the way out, and all the way back in. It seemed to get the closest when I went all the way in, but it never got focused - not even close!

My setup that did not achieve focus. I think it's too long, so I'm going to cut out some pieces.

I had my t-adapter nested into my 1.25" eyepiece holder because that's the only way that everything seemed to fit together. I tried taking my diagonal mirror off, and my other bits and pieces of tube and connectors - what a mess! I was trying to find a way to shorten my setup, because I figured that would compensate for my inability to focus any further 'up' the focuser tube.

Venus out of focus at the limit of my focuser (butting up against optical tube)

I finally looked up my telescope manual and read that my focuser tube is .965", which seems to be a smaller but somewhat common dimension for these low power scopes. So the issue is that my 1.25" thread t-adapter barrel (aka female) won't screw directly onto my .965" thread (aka male) focuser tube.

I also got some feedback on Instagram that I should get a Barlow lens, because not only will it give me about 2x magnification, but it will change my prime focus distance from the optical tube to within a range that my focuser can reach. I think I'll hold off on that until this guy comes in the mail. I'm not sure where a Barlow lens would fit exactly, and if they make a .965" to t-thread Barlow. I feel like I'm playing with a really expensive LEGO set and the pieces aren't matching up!

As a consolation prize, however, I'm pretty sure my out of focus image of Venus corresponds to the phase of Venus last night, so that's pretty neat.

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